"A file extension is an identifier used as a suffix to a name of the computer file in an operating system such as Microsoft Windows so that it can recognize the program to open much easily The file extension for a Microsoft Excel 2010 file is .xlsx The older file extension for the 97-2003 worksheet was .xls    Program Extension Word .docx PowerPoint .pptx Printing XML document .xps  "

"The correct answer is .xltx Key Points .xltx: An XLTX file is a template created by Microsoft Excel, a spreadsheet application included in the Microsoft Office suite.  It contains predefined settings and layout information used to create multiple Excel spreadsheets (typically . XLSX files) with similar formatting and properties. The XLTX format replaced the older XLT template format from Office 2007 and earlier. Additional Information .xlsm: An XLSM file is a macro-enabled spreadsheet created by Microsoft Excel, a widely-used spreadsheet program included in the Microsoft Office suite.  It contains worksheets of cells arranged by rows and columns as well as embedded macros programmed in the Visual Basic for Applications (VBA) language.  XLSM files are saved in the Open XML format introduced in Microsoft Office 2007. .xlsx: An XLSX file is an Excel spreadsheet created by Microsoft Excel or exported by another spreadsheet program, such as OpenOffice Calc or Apple Numbers. It stores data in worksheets, which contain cells arranged in a grid of rows and columns, and may also contain charts, mathematical functions, styles, and formatting. "

Explanation:xls is a file extension that is used to save the file is Microsoft Excel 2010​.

Microsoft Excel 2010 supports the use of an XML-based file format called the Microsoft Office Open XML format. This file format carries the filename extension . xlsx for Excel workbooks and . xlsm for macro-enabled workbooks.